Method and apparatus for uniformly baking substrates such as photomasks

ABSTRACT

A method and apparatus for baking a film onto a substrate. A film, such as a layer of photoresist, is disposed on a first surface of a substrate while a second surface is exposed to a liquid bath. The liquid bath is maintained at a pre-selected temperature. Exposure of the substrate to the liquid bath allows the film on the opposite surface to bake. The liquid bath is then re-circulated to maintain a constant and uniform temperature gradient across the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to semiconductor fabrication.More particularly, the present invention relates to the process ofphotolithography, and even more specifically to baking a photoresistonto a substrate, such as a photomask, during the photolithographyprocess.

2. Background of the Related Art

This section is intended to introduce the reader to various aspects ofart which may be related to various aspects of the present inventionwhich are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

During fabrication of a semiconductor device, a process calledphotolithography is employed to create patterns on a semiconductivesubstrate, commonly known as a wafer. The wafer is built up one layer ata time creating an overlay of complex patterns which ultimately formelectrical devices and paths. The photolithography process comprises avariety of steps to accomplish the patterning of each layer. One commonstep of the process involves the use of one or more photomasks. Eachphotomask has a pattern formed thereon, and this pattern is transferredonto a semiconductor wafer by irradiating the wafer through thephotomask.

To fabricate a photomask, surface preparation is required wherein asurface of the substrate, often referred to as a photomask blank, iscleaned and dried. The substrate may be composed any of a number ofmaterials. For example, the photomask blank may be made of glass orquartz. Surface preparation of the substrate is done in anticipation ofa photoresist being applied to the substrate's surface. The photoresistrequires a clean and dry surface in order to accomplish a high level ofadhesion. The photoresist is typically applied by placing a coat of thematerial on the substrate and then spinning the substrate to obtain athin uniform film across the blank's surface. After the photoresist hasbeen applied, the blank and film are subjected to a first baking processoften referred to as soft bake or pre-bake. The soft bake process servesto evaporate a portion of the photoresist solvents. Besides removingunwanted solvents, which will interfere with subsequent processing, thesoft bake also helps to facilitate better adhesion between thephotoresist and the substrate. After the soft bake the photoresistremains as a relatively soft coating on the substrate.

The baking process may be accomplished using various methods. Two of themore typical methods include use of a hot plate or the use of aconvection oven. In the hot plate method, the substrate is placeddirectly on the hot plate for heating by conduction. Heat is transferredfrom the hot plate to the photomask blank and then through the blank tothe photoresist layer. This technique provides good temperature controland allows for small batches to be processed simultaneously.

Alternatively, the convection oven method utilizes a fluid medium,usually a gas such as air, to heat the substrate and film. Convectionbaking allows for a more direct baking of the photoresist layer sinceconvection baking does not have to rely on conduction through thesubstrate. Convection ovens permit large batches of photomasks to beprocessed at one time, but typically these ovens have inferiortemperature control in comparison with the hot plate method. Convectionovens also typically take longer per batch to process than do hotplates. Other alternative methods for baking include microwave,infra-red, and vacuum oven baking.

Various sources of exposure, including optical sources, x-rays or ionbeams, may be used for exposing the photoresist. The exposure causes achemical reaction to take place in the photoresist layer. For example,in one type of photoresist, exposure causes a polymerization of thephotoresist. Thus, by using a mask and an exposure source, a pattern ofpolymerized resist (and a mating pattern of non-polymerized resist) isformed on the surface of the wafer. This process, while described abovein general terms, is actually rather complex and involved. Likewise,there are various exposure sources to choose from, each with its ownadvantages and complexities. Also, there are multiple types ofphotoresist. Each type of photoresist has different characteristics andresponds differently to the various manipulative steps in thephotolithography process.

After the film of photoresist has been exposed, the photoresist is thendeveloped. Developing is a chemical process wherein chemical dissolutionof unpolymerized regions in the photoresist occurs. Different developingchemicals and techniques are often employed depending on the type ofphotoresist being used. After the photoresist has been developed, thechemical is rinsed off and the substrate is allowed to dry. Polymerizedregions of the photoresist remain on the surface of the substrate. Afterdeveloping of the photoresist, the substrate may optionally undergo asecond baking process. The second baking process, often referred to ashard bake, again serves to evaporate remaining solvents in thephotoresist and to create better adhesion of the photoresist to thesubstrate. The methods and techniques used for hard baking areessentially the same as those used for soft baking.

While described in generalities above, the process for fabricatingphotomasks is complex and requires careful attention to many details.Mistakes and errors can be introduced at any step of the process causingresultant defects in the final product. Likewise, each step of theprocess is continually scrutinized for possible improvements. One areawhere improvement is contemplated is in the baking processes. A gooddeal of variability may be introduced into the process during the bakingsteps. For example, it has been noted that the temperature gradientfound in a substrate during baking is not uniform. This means that thetemperature at the outside edge of the substrate is not the same as thetemperature at the center of the substrate. Often the range of thetemperature gradient is several degrees. The variation of temperatureresults in uneven baking of the photoresist layer. The uneven baking canlead to poor performance of the photoresist layer during the exposureand developing steps. For example, the lines formed in the photoresistlayer during exposure and developing can vary in width depending ontheir location on the photomask blank. A region of the photoresist layerbaked at the desired temperature will produce lines at a predictedwidth, however, a region of the photoresist layer baked at the variedtemperature will produce lines which vary from the predicted width. Thusthe precision of the exposed image becomes a function of the temperaturegradient experienced by the photoresist layer.

In consideration of the heating methods employed, numerous factorsaffect the resultant baked film. For example, one problem with the hotplate method of baking is that the surface of the hot plate may not beco-incidentally parallel with the surface of the substrate being baked.The result of the two surfaces not being exactly parallel is air gapspresent between the two surfaces. Since the hot plate method of bakingis a process of conduction, the air gaps create an inefficiency becauselocalized regions experience heat transfer by convection instead ofconduction. The conduction transfers the heat to the substrate muchquicker than does the convection in the air gaps. Therefore, heat isunevenly distributed to the surface of the substrate from the hot plate.Again the ultimate result is an undesired temperature gradient in thesubstrate and non-uniform baking of the photoresist layer.

Another problem associated with hot plate baking is the transientformation of temperature zones in the hot plate. This is often theresult of a rapid temperature spike in the heating element. The heatingelement attains a specific temperature and then the surrounding materialtries to attain the same temperature as the heating element. Simplystated, the hot plate is trying come to an equilibrium temperature, butin the process temperature zones are created. These temperature zonesare transient, but can result in similar temperature zones beingtransferred to the substrate and film. As an example, on study has foundthat, depending on the particular steps and methods followed, hot platebaking may result in temperature variations of from 3° to 6° C. over a132 mm square area.

While convection ovens generally do a better job in respect tominimizing the production of temperature zones, there are opportunitiesfor improvement. For example, the medium used for heat transfer inconvection ovens is typically air. Convection by air is not as efficientas the hot plate method of conduction. The relative inefficiency of airas heat transfer medium is one reason why the convection method istypically slower than the hot plate method.

The present invention may address one or more of the problems set forthabove.

SUMMARY OF THE INVENTION

Certain aspects commensurate in scope with the originally claimedinvention are set forth below. It should be understood that theseaspects are presented merely to provide the reader with a brief summaryof certain forms the invention might take and that these aspects are notintended to limit the scope of the invention. Indeed, the invention mayencompass a variety of aspects that may not be set forth below.

In accordance with one aspect of the present invention, a method ofbaking a photomask is provided. The method includes providing asubstrate having two surfaces wherein a film, such as a layer of photoresist, is disposed on one of the surfaces. The uncoated surface is thenexposed to a temperature controlled liquid bath. Exposure of thesubstrate to the liquid bath is maintained for a predetermined time toallow the film to bake. Re-circulation of the liquid bath maintains asubstantially constant temperature gradient across the substrate.

In accordance with another aspect of the present invention, an apparatusand a related system is provided to allow a film to be baked on asubstrate more uniformly. The apparatus includes a first tank forcontaining a liquid bath. The first tank may be disposed inside a secondtank to help facilitate re-circulation of the liquid bath. The substrateis partially disposed within the first tank having a surface exposed tothe liquid bath. A re-circulation system is provided to circulate theliquid bath against the exposed surface of the substrate. There-circulation system also allows for the liquid to be exposed to atemperature controlling unit such as a heat exchanger.

In accordance with yet another aspect of the present invention, a systemfor uniformly baking a film on a substrate is provided utilizing manyfeatures of the disclosed apparatus. The system takes advantage of thesefeatures to provide batch baking of the substrates.

DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 illustrates a sectional view of one embodiment according to thepresent invention;

FIG. 2 illustrates a sectional view of an alternative embodimentaccording to the present invention;

FIG. 3 illustrates a schematic of an alternative embodiment of theinvention;

FIG. 4 illustrates a schematic of another alternative embodiment of theinvention; and

FIG. 5 illustrates a sectional view of another alternative embodiment ofthe invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

Turning now to the drawings, and referring initially to FIG. 1, a bakingapparatus 10 according to the present invention is shown. The bakingapparatus 10 includes a primary tank 12 which is formed with a wall 14that defines an upper limit, or a fluid level, for the primary tank 12.The primary tank 12 is disposed within a secondary tank 16. A liquidbath 18, such as propylene glycol, is held in the primary tank 12 andfills the primary tank 12 to capacity.

A retaining device 20 is partially disposed within the primary tank 12and holds a substrate 22. The substrate 22 is contemplated as being aphotomask, a flat panel display, or any other suitable substrateundergoing a bake process. The substrate 22 has a first surface 24 uponwhich a film is disposed (not shown). The film disposed on the substrate22 is a photoresist film which is utilized in a photolithographyprocess. However, it is noted that for other applications the film maybe formed of a different material. The substrate 22 also has a secondsurface 26. The substrate 22 is placed in the retaining device 20 suchthat the second surface 26 is exposed to the liquid bath 18.

Attached to the secondary tank 16 is a return line 28. The return line28 accommodates the flow of any excessive liquid bath 18 which flowsover the wall 14 of the primary tank 12. The return line 28 is coupledto a circulating device 30 such as a pump. A supply line 32 is coupledto the pump 30 and carries the liquid bath 18 back to the primary tank12. A temperature control unit 34, such as a heat exchanger, is disposedadjacent the supply line 32. While the circulating device 30 and thetemperature control unit 34 are shown as being separate devices in FIG.1, they may be combined into a single unit. The temperature control unitmay have a broad temperature range, such as 80° to 200° C., with apreferred operating range of 100° to 120° C., and precise temperaturecontrol such as ±0.05 to 0.1° C. Of course the upper level temperaturerange will have some limit placed upon it depending on the fluid usedfor the liquid bath.

The basic operation of the baking apparatus 10 will now be describedwith reference to the fluid flow lines 36 as shown in FIG. 1. Startingat the pump 30, the liquid bath 18 is pumped through the supply line 32where it is heated to a pre-selected temperature by the temperaturecontrol unit 34. The liquid bath 18 then flows into the primary tank 12.Advantageously, a baffle 38 or series of baffles are used to control theflow of the liquid bath 18 and properly disperse it as it enters theprimary tank 12. The liquid bath 18 then circulates through the primarytank 12. The flow of the liquid bath 18, as generally shown by fluidflow lines 36, is upward and radially outward from the center of theprimary tank 12. The wall 14 of the primary tank 12 serves as a weir, ora simple fluid level control, allowing excess fluid to spill over in thesecondary tank 16. The excess fluid collects in the secondary tank 16and flows through the return line 28 and back to the pump 30 to bere-circulated through the system again.

The baking apparatus 10 allows the second surface 26 of the substrate 22to be exposed to a circulating liquid bath 18 which is being controlledto a substantially constant temperature as described above. The heatsupplied to the second surface 26 is conducted through the substrate 22to bake the film disposed on the first surface 24 substantiallyuniformly. Because the second surface 26 is exposed to the liquid bath18, a fluid and compliant medium, there are no gaps such as thoseassociated with a typical hot plate. Also, because the heat transferprocess of the system is forced convection, there is a reduction oftemperature zones as experienced with a conductive hot plate. Incomparison with a convectional oven utilizing air as a medium, theliquid bath 18 offers a more controlled and efficient medium ofconvection. In sum, substrate 22 is exposed to a more uniform andefficient heat transfer process.

Turning now to FIG. 2, an alternative embodiment is disclosed. Thebaking apparatus 40 includes a tank 42. The tank 42 may be similar tothe primary tank disclosed in FIG. 1. The tank 42 contains a liquid bath44. Unlike the embodiment of FIG. 1, the tank 42 is not filled tocapacity with the liquid bath 44. The liquid bath 44 may be any of anumber of suitable liquids such as propylene glycol, ethylene glycol, orpetroleum based or synthetic oils. It is important however, that theliquid bath be compatible with any resist being used to form a film onthe substrate 48.

A retaining device 46 is disposed within the primary tank 42 andpartially disposed within the liquid bath 44. The retaining device 46positions and holds a substrate 48 within the tank 42. The substrate 48is contemplated as being a photomask, a flat panel display, or any othersuitable substrate undergoing a baking process. The substrate 48 has afirst surface 50 upon which a film (not shown) has been disposed. Thefilm disposed on the substrate 48 may be a photoresist film utilized ina photolithography process. However, it is noted that for othersubstrates the film may be formed of a different material. The substrate48 is placed in the retaining device 46 such that a second surface 52 isexposed to the liquid bath 44.

Attached to the tank 42 is a temperature control unit 54, such as a heatexchanger. The temperature control unit 54 is shown as being disposed atthe bottom of the tank 42. The temperature control unit 54 is used tocontrol the temperature of the liquid bath 44. In the embodiment shown,the temperature control unit 54 is placed so as to conduct heat throughthe tank 42 to the liquid bath 44. However, it is contemplated thatalternative arrangements could be utilized for controlling thetemperature of the liquid bath 44.

In operation, the temperature control unit 54 heats the liquid bath to adesired temperature and maintains the desired temperature within ±0.05to 0.1° C. Because the temperature control unit 54 is placed at thebottom of the tank 42, the liquid bath 44 at the bottom of the tank 42heats first. The liquid bath 44 at the bottom of the tank 42 becomesless dense as it rises in temperature and so begins to rise to the topof the tank 42. Liquid bath 44 towards the top of the tank, having aslightly lower temperature, circulates to the bottom of the tank becauseof its higher density. This circulation pattern is known as naturalconvection. The circulation of the liquid bath 44 is generally indicatedby the flow lines 56. The liquid bath 44 flows in a manner which allowsconstant convectional heat transfer from the liquid bath 44 to thesecond surface 52 of the substrate 48.

In sum, the baking apparatus 40 as described allows the second surface52 of the substrate 48 to be exposed to a circulating liquid bath 44.The heat supplied to the second surface 52 is conducted through thesubstrate 48 to bake the film disposed on the first surface 50substantially uniformly. Because the second surface 52 is exposed to theliquid bath 44 there are no air gaps as found in a typical hot plate.The entire substrate is subject to the same heat transfer process with aresulting substantially uniform temperature gradient.

Turning now to FIG. 3, an alternative baking system 60, based on thegeneral embodiment disclosed in FIG. 1, will now be discussed. Acombined circulation/temperature control unit 62 is connected to asupply line 64. The supply line 64 carries liquid bath to a plurality ofprimary tanks 66. The primary tanks 66 are considered to be similar tothat shown in FIG. 1. The primary tanks 66 are filled with a liquid bathto capacity (not shown). Similar to the configuration shown in FIG. 1, asubstrate 68 is disposed so that a bottom surface of the substrate isexposed to the liquid bath. Excess liquid bath is carried over the wallsof the primary tanks 66 into a common secondary tank 70. The secondarytank 70 is connected to a return line 72 which conveys the excess liquidbath back to the circulation/temperature unit 62 to be processed onceagain. The general flow of the liquid bath is indicated by the flowlines 74.

The baking system 60 operates in a similar manner to the bakingapparatus 10 disclosed above, however, the baking system 60 is designedmore particularly for batch operations. It is noted that in the bakingsystem 60, a single supply line proceeds from thecirculation/temperature unit 62 but later divides into individual linesfor each primary tank 66. The individual lines are shown to be inparallel with one another, thus a distribution of liquid bath havinguniform temperature is provided to each primary tank 66.

FIG. 4 discloses an alternative baking system 80 for batch operations. Acombined circulation/temperature control unit 82 is connected to asupply line 84. The supply line 84 carries liquid bath to a primary tank86. The primary tank 86 is, in general, considered to be similar to thatshown in FIG. 1 with the difference of the primary tank 86 facilitatingmultiple substrates 88. The primary tanks 86 is filled with a liquidbath to capacity (not shown). Multiple substrates 88 are disposed sothat the bottom surface of each substrate is exposed to the liquid bath.Again, excess liquid bath is carried over the wall of the primary tank86 into a secondary tank 90. The secondary tank 90 is connected to areturn line 92 which conveys the excess liquid bath back to thecirculation/temperature unit 82 to be processed once again. The generalflow of the liquid bath through the system is indicated by the flowlines 94.

The baking system 80 operates similarly to the baking system 60discussed above. However, the baking system 80, as shown in FIG. 4,utilizes a single primary tank for heating multiple substrates. In thebaking system 80, a single supply line proceeds from thecirculation/temperature unit 82 but later divides into multiple feedlines before connection with the primary tank 86. The individual supplylines are shown to be in parallel with each other and can be designed torender a specific distribution and flow of the liquid bath within theprimary tank 86. The desired flow and distribution of the liquid bathmay be accomplished in various ways. For example, proper baffleconfigurations and/or strategic locations of supply line inlets willhelp to create a proper flow and distribution of the liquid bath.

It should be noted that various modifications and alternativeembodiments are contemplated as being within the scope of the invention.For example, the retaining device used to hold the substrate may be aconductive or insulative element depending on the overall design of theapparatus or system. Also, the retaining device may be formed of asingle continuous unit for each substrate, multiple elements persubstrate, or one unit which will accommodate multiple substrates. Theretaining device may be coupled to one of the tanks, or it may be acomponent of separate but related automation equipment used to place thesubstrate into the baking apparatus or system.

It is also contemplated that in the embodiments using a re-circulationsystem a secondary tank may not be utilized. Instead, a drain oroverflow could be provided in the primary tank with appropriate pipingto connect the overflow with the pump system. Such an embodiment isdisclosed in FIG. 5, showing a baking system 100 utilizing an overflowdevice 102 in the primary tank 104. The overflow device 102 is connectedto a circulation system 106 by means of a return line 108. The overalloperation of the baking system 100 remains the same as disclosed above.Of course multiple overflow devices could be implemented, each beingstrategically placed for additional flow control. The secondary tankdisclosed in the embodiment of FIG. 1 is, in actuality, a continuousoverflow located around the periphery of the primary tank.

Also, while the disclosed embodiments all have been disclosed in termsof baking a film onto a substrate, and thus heating of the substrate hasbeen contemplated, the embodiments may be utilized for cooling orquenching of a substrate or other similar work piece. The temperaturecontrol unit would thus act to cool the liquid bath rather than heat it.The liquid bath would then cool the surface of the substrate byconvection. It is noted that such an embodiment would be more readilyaccomplished with a forced convection device such as that disclosed inFIG. 1 than with a natural convection device such as that shown in FIG.2.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

What is claimed is:
 1. A method of baking a substrate comprising: providing a substrate having a first surface wherein a film is disposed on the second surface; exposing only the second surface to a liquid bath; controlling the temperature of the liquid bath; maintaining the exposure of the second surface to the liquid bath for a predetermined time to allow the film to bake on the first surface; and subjecting the film to an independent secondary heat source simultaneously with the exposure of the second surface to the liquid bath.
 2. The method of claim 1, wherein the substrate is a photomask.
 3. The method of claim 1, wherein the temperature of the liquid bath is controlled to ±0.05 to 0.1° C.
 4. The method of claim 1, wherein the substrate is a flat panel display.
 5. The method of claim 1, wherein the liquid bath is circulated by natural convection.
 6. The method of claim 1, comprising circulating the liquid bath adjacent the second surface of the substrate.
 7. The method of claim 6 wherein controlling the temperature of the liquid bath comprises re-circulating the liquid bath through a heat exchanger.
 8. The method of claim 1, wherein the film comprises a layer of photoresist.
 9. The method of claim 1, wherein the liquid bath comprises propylene glycol.
 10. A method of baking a substrate comprising: providing a substrate having a first surface and a second surface wherein a film is disposed on the first surface; exposing the second surface to a liquid bath; controlling the temperature of the liquid bath; maintaining the exposure of the second surface to the liquid bath for a predetermined time to allow the film to bake on the first surface; and subjecting the film to an independent secondary heat source simultaneously with the exposure of the second surface to the liquid bath.
 11. The method of claim 10, wherein the substrate is a photomask.
 12. The method of claim 10, wherein the temperature of the liquid bath is controlled to ±0.05 to 0.1° C.
 13. The method of claim 10, wherein the substrate is a flat panel display.
 14. The method of claim 10, wherein the liquid bath is circulated by natural convection.
 15. The method of claim 10, comprising circulating the liquid bath adjacent the second surface of the substrate.
 16. The method of claim 15 wherein controlling the temperature of the liquid bath comprises re-circulating the liquid bath through a heat exchanger.
 17. The method of claim 10, wherein the film comprises a layer of photoresist.
 18. The method of claim 10, wherein the liquid bath comprises propylene glycol. 